The Rick Brant adventure novels are scarce and tend to be priced for collectors. I hadn’t realized until just now, however, that nearly half of them have fallen into the public domain and are available through Project Gutenberg. Cleaned-up versions are also available at Manybooks.
I’m pleased that this includes the first one I read, The Egyptian Cat Mystery, which does an excellent job of introducing the real science of SETI, unlike, say, every other boy’s adventure novel I’ve ever seen that dealt with aliens. Why? Wikipedia says, “During the 1960s, Goodwin served as Special Assistant to the Administrator of NASA…”